1. Field of Invention
The invention relates to a energy balance circuit and a charging bypass circuit for batteries and, in particular, to a energy balance circuit and a charging bypass circuit that can equally charge a set of batteries connected in series.
2. Related Art
Many applications require the use of many batteries connected in series. However, due to different characters and residual energy in batteries, they have to be properly matched. If the residual energy in the batteries connected in series is different, those with more residual energy are likely to be overcharged and damaged while those with smaller residual energy are not charged to full when they are charged in series. When discharging, those batteries not charged in full are likely to overdischarge to damage the batteries. Therefore, how to avoid such problems is an important issue for serial battery sets.
To protect the battery set, the simplest method is to connect to each battery a resistor and a control switch in parallel. FIG. 1 shows an example with a battery set of three batteries. It includes a charging circuit CHR. The battery set is comprised of a first battery B1, a second battery B2, and a third battery B3 connected in series. The first battery B1 is connected in parallel a first resistor R1 and a first switch SW1. The second battery B2 is connected in parallel a second resistor R2 and a second switch SW2. The third battery B3 is connected in parallel a third resistor R3 and a third switch SW3.
When charging, if the terminal voltage of any battery reaches a predetermined value, the corresponding switch is turned on. Suppose the voltage of the second voltage B2 reaches the predetermined value, the second switch SW2 is turned on. The second resistor R2 can be shown as connected to the second battery B2 in parallel. In such a way, the current originally flowing through the second battery B2 is reduced because part of it flows through the second resistor R2. This can avoid overcharging.
The circuit shown in FIG. 1 produces heat as the current flows through the resistor. Therefore, it cannot process large branch currents.
To solve such problems, the U.S. Pat. No. 5,479,083 improved the above-mentioned dissipative method and provided with a non-dissipative energy balance circuit. The circuit structure is shown in FIG. 2, including a first battery B1 and a second battery B2 connected in series, and a first switch SW1 and a second switch SW2 connected in series. The battery set and the switch set are connected in parallel. One end of an inductor L is coupled between the first battery B1 and the second battery B2, the other end is coupled between the first switch SW1 and the second switch SW2. The basic principle is that the first switch SW1 and the second switch SW2 are turned on and off in an alternative way so that the terminal with a higher voltage discharges while that with a lower voltage is charged in this circuit. When the first switch SW1 is on, the first battery B 1 and the inductor L form a loop. When the second switch SW2 is on, the second battery B2 and the inductor L form a loop. Therefore, the bypass charging/discharging current is of pulse nature.
FIG. 3 illustrates another energy balance circuit with the similar idea of FIG. 2, which is disclosed in the U.S. Pat. Nos. 6,150,795 and 6,222,344. The switches SW1 and SW2 are on and off simultaneously, or only the switch corresponding to the battery with a larger terminal voltage is activated. If the terminal voltage of the first battery B1 is larger, the first switch is turned on and off at a high frequency. In this way, the energy in the first battery B 1 is transferred to the second battery B2 through the circuit. The energy transfer is possible only when the first switch SW1 and the second switch SW2 are on. Its transfer method is different from that of the U.S. Pat. No. 5,479,083. However, it also has pulse currents.
The U.S. Pat. No. 5,659,237 also discloses an energy balance circuit that distributes a total energy in an even way. Its main technical feature is to redistribute the energy in a battery set through a circuit to each of the batteries. Batteries with smaller terminal voltages get more energy while those with larger terminal voltages get less energy. Therefore, this circuit can achieve the goal of making the terminal voltage of each battery in the battery set the same.
The transformer in the U.S. Pat. Nos. 6,008,623 and 5,659,237 is changed to several independent ones. The basic idea of the U.S. Pat. No. 5,666,041 is the same as the U.S. Pat. No. 5,659,237. It also redistributes the serial battery set energy. The only difference is in the structure of the transformer. In order for the batteries with smaller terminal voltages to be given more energy, the U.S. Pat. No. 5,982,143 further includes a switch connected in series in front of a diode.
The battery energy balance circuit disclosed in the prior art few has the modularized property. When the number of battery sets increases, the design of the whole balance circuit has to be modified and the number of windings has to change too. Therefore, it is not economical in practice. Moreover, the charging and discharging currents in the circuit of the prior art are all pulse currents.